The presence of sulfur containing compounds in hydrocarbon streams is a major cause for concern in the hydrocarbon industry. The presence of sulfur containing compounds such as carbon disulfide, carbonyl sulfide, hydrogen sulfide, mercaptans, sulfides, and disulfides is not desirable in hydrocarbon streams, more particularly in propylene containing hydrocarbon streams as the presence of these sulfur impurities poison/contaminate the catalyst used in propylene polymerization. Furthermore, these sulfur impurities also impart odor to the polypropylene product. The cumulative concentration of sulfur impurities that can be tolerated in a propylene stream is less than 0.5 ppm; more preferably less than 0.1 ppm. The Hydrocarbon industry has adopted various methods for the removal of sulfur impurities from hydrocarbon streams such as alkali treatment, amine treatment, use of regenerable adsorbents having alkali/alkaline earth metal/metal oxide impregnated on alumina support and use of CuO/ZnO, Ni/NiO based non-regenerable adsorbents.
The removal of sulfur impurities from hydrocarbon streams using non-regenerable adsorbents has been disclosed in several patents. However, in due course, use of regenerable spent adsorbents has gained a lot of significance, in order to make the sulfur capturing process more economical and efficient on a commercial scale. There are several patents which describe the processes for regeneration of sulfur capturing adsorbents.
Existing Knowledge:
Following patents disclose the processes for regeneration of sulfur capturing adsorbents, utilized for removal of sulfur impurities from hydrocarbon streams. U.S. Pat. No. 4,835,338 discloses a process for the removal of COS present in a liquid hydrocarbon stream using metal impregnated activated alumina. The spent adsorbent is regenerated in a column by passing a dry gas such as air, hydrocarbons gases, nitrogen or other inert gases through the adsorbent, in the temperature range of 150° C. to 300° C. The regeneration efficiency of the adsorbent decreases drastically after each regeneration cycle and after 3 regenerations, the regeneration efficiency is reduced to 30% of the initial capacity.
U.S. Pat. No. 5,157,201 teaches a process for the removal of sulfur components from olefinic hydrocarbon streams containing propylene and propane, by using hydrodesulfurization catalyst such as CoMo, NiMo, and Ni supported on alumina, in the absence of extraneously added hydrogen. The regeneration of the spent adsorbents is carried out by using air or a mixture of air, nitrogen and steam at a temperature in the range of 400° C. to 500° C.
U.S. Pat. No. 6,843,907 reveals a process for the removal of COS from hydrocarbon streams using alumina supported Na2O as an adsorbent. It also discloses a regeneration process for the spent adsorbent in the presence of heated gas containing water as a hydrolyzing agent. The regeneration is carried out at lab scale under isothermal conditions using small amounts of adsorbent. However, it is document is silent on the information regarding the exotherm generated while introducing hydrolyzing agent during the regeneration process.
One of the severe shortcomings of the known processes for regenerating the spent sulfur capturing adsorbent under adiabatic conditions using a hydrolyzing agent is that it leads to excessive exotherm generation. The excessive exotherm generation reduces the sulfur loading capacity of the regenerated adsorbents. Furthermore, it also results in the loss of adsorption activity of sulfur capturing adsorbents. There therefore exists a need for a process which is suitable for commercial scale regeneration of the sulfur capturing agent which minimizes the loss in adsorption efficiency of the adsorbent, ensures the longevity of the adsorbent by controlling the excessive exothermic generation.